doi:10.1016/S0016-7037(00)00092-9

Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA1240): An HED parent body impact melt

J. A. BARRAT,1,* A. JAMBON,2 M. BOHN,3 J. BLICHERT-TOFT,4 V. SAUTTER,5 C. GOPEL,6 PH. GILLET,4 O. BOUDOUMA,7

and F. KELLER8

1CNRS UMR 6112 (Ge´odynamique et Plane´tologie) and Universite´ d’Angers, 2 bd Lavoisier, 49045 Angers Cedex, France2Laboratoire Magie (CNRS UMR 7047), Universite´ Pierre et Marie Curie, case 110, 4 place Jussieu, 75252 Paris Cedex 05, France

3Ifremer-Centre de Brest, (CNRS-UMR 6538), BP70, 29280 Plouzane´ Cedex, France4Laboratoire des Sciences de la Terre, CNRS UMR 5570, Ecole Normale Supe´rieure de Lyon, 46 Alle´e d’Italie, 69364 Lyon Cedex 7, France

5Museum National d’Histoire Naturelle, Laboratoire de Mine´ralogie, CNRS FR 32, 61 rue Buffon, 75005 Paris, France6Institut de Physique du Globe de Paris, Laboratoire de Ge´ochimie et Cosmochimie (CNRS-UMR 7579), 4 place Jussieu,

75252 Paris Cedex 05, France7UniversitePierre et Marie Curie, U.F.R. Sciences de la Terre, case 110, 4 place Jussieu, 75252 Paris Cedex 05, France

8CNRS UMR 5025-UJF, Maison des Ge´osciences, 1381 rue de la Piscine, 38400 Saint Martin d’He`res, France

(Received July 5, 2002;accepted in revised form December 26, 2002)

Abstract—NWA 1240 is an unusual eucrite recently recovered in Morocco as a single stone of 98 g. It is anunbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting ofskeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongatepyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minuteamounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges ofcomposition so far described for a eucrite, from En76.0Wo1.9Fs22.1 to compositions nearly devoid of Mg(unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals containsignificant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe2�)Si3O8 plagioclasecomponent.

To discuss the potential effects of hot-desert weathering on NWA 1240, we have analyzed a series ofSaharan eucrites (Agoult, Aoufous, Igdi, Smara, NWA 047 and NWA 049) and large aliquots (0.39 to 2.8 g)of eucrite falls (Bereba, Bouvante, Jonzac, Juvinas and Serra de Mage´). These results indicate that among theelements we have determined, Pb, Ba and Sr are the most sensitive indicators of Saharan weathering.

The bulk composition of NWA 1240 has been determined for 45 elements by ICP-AES and ICP-MS. Thedata show that the meteorite is not significantly weathered: its Pb concentration is very low; Ba and Srconcentrations are not anomalously high; the Th/U and Hf/Sm ratios are chondritic (Th/U� 3.65, Hf/Sm�0.74). NWA 1240 is rich in MgO (10.4 wt%) and Cr2O3 (0.71 wt%), and displays striking similarities withcumulate eucrites, such as having similar incompatible trace element patterns and a significant positive Euanomaly (Eu/Eu*� 1.37). The combination of fast cooling and cumulate eucrite-dominated compositionsuggests that NWA 1240 is not an igneous rock but rather an impact melt.Copyright © 2003 Elsevier Ltd

1. INTRODUCTION

The howardite, eucrite and diogenite (HED) meteorites makeup the most common suite of achondrites and represent approx-imately 2–3% of the current meteorite population (Shearer etal., 1998). These rocks are among the oldest evidence ofigneous activity in the Solar System (�4.5 Ga, e.g., Nyquist etal., 1997; Tera et al., 1997; Lugmair and Shukolyukov, 1998;Yamaguchi et al., 2001, and references therein) and are be-lieved to have originated on the same large asteroid, possibly4-Vesta (e.g., Drake, 2001). Diogenites are cumulate rocksconsisting essentially of orthopyroxene, whereas eucrites areplagioclase- and pigeonite-bearing basalts or gabbros. Howard-ites are polymict breccias composed of both of these litholo-gies.

Most of the HED meteorites have recorded complex post-

crystallization histories (e.g., Metzler et al., 1995; Yamaguchiet al., 1996, 2001). The surface of the HED parent body wasextensively brecciated and locally melted by meteorite impacts.Therefore, HEDs are usually breccias containing one or moretypes of clasts. Unbrecciated HEDs, such as the vesiculareucrite Ibitira, the cumulate eucrites Moore County and Serrade Mage, and the diogenites Tatahouine and GRO95555, arerare. Most of the HED meteorites, including the scarce unbrec-ciated ones, have experienced extensive thermal annealing,producing recrystallization textures and exsolution in py-roxenes. Only exceptionally have unmetamorphosed (or un-equilibrated) lithologies been preserved, and these are mainlysmall-size objects (e.g., Bluewing 001, Warren and Gessler,2001) or fragments in some polymict eucrites (e.g., Y7501,Takeda et al., 1994).

The petrogenesis of eucrites and diogenites is a long-stand-ing controversial issue for which a variety of models have beenput forward (e.g., Stolper, 1977; Jurewicz et al., 1993; Righterand Drake, 1997; Ruzicka et al., 1997; Takeda, 1997; Warren,1997; Mittlefehldt et al., 1998; Barrat et al., 2000; Drake, 2001;Blichert-Toft et al., 2002). One of the most critical unresolved

* Author to whom correspondence should be addressed at presentaddress: 1 CNRS UMR 6112 (Ge´odynamique and Plane´tologie) andCNRS UMR 6538 (domaines oce´aniques), I.U.E.M., Universite´ deBretagne Occidentale, Place Nicolas Copernic, F 29280 Plouzane´,France (barrat@univ-brest.fr).

Pergamon

Geochimica et Cosmochimica Acta, Vol. 67, No. 20, pp. 3959–3970, 2003Copyright © 2003 Elsevier Ltd

Printed in the USA. All rights reserved0016-7037/03 $30.00� .00

3959

questions concerns the nature of the primary melts produced bythe igneous activity on the HED parent body. It has beensuggested that the so-called “main group eucrites” and “Stan-nern trend eucrites” are near-primary melts (e.g., Stolper,1977), but this interpretation is not a consensus. The parentalmagma of the diogenites and its possible relationship witheucritic melts is an even more enigmatic subject. None of theknown HEDs, including high-Mg eucrites, represent such amelt. Most of the high-Mg eucrites probably represent impactmelts rather than true igneous products (Mittlefehldt and Lind-strom, 1997). Nevertheless, the existence of high-Mg eucrites isencouraging because although some may represent rare impact-melted lithologies from the HED parent body, some of themcould be the “missing link” between eucrites and diogenites.

Northwest Africa 1240 (NWA 1240), found in November2001 in Moroccan Sahara, is an unusual eucrite. It is unusual inthat it is unbrecciated, exhibits textures similar in many aspectsto some lunar mare basalts, displays one of the widest ranges inunequilibrated pyroxene compositions so far reported for aeucrite, and is unusually Mg-rich. Here we report on the pe-trology and geochemistry of NWA 1240. We also present datafor 13 aliquots of 6 other Saharan eucrite finds and 8 largealiquots of 5 eucrite falls to assess the effects of hot desertweathering on eucrites.

2. ANALYTICAL METHODS

Petrographic observations and quantitative chemical analyses of thevarious mineral phases of NWA 1240 were made on a polished sectionof �2 cm2. Backscattered electron (BSE) images were taken withJEOL 840-A and JEOL JSM6301-F scanning electron microscopeslocated, respectively, at Universite Pierre et Marie Curie, Paris, andSCIAM (Service Commun d’ Imagerie et d’Analyse Microscopiques),Angers. Electron microprobe analyses were carried out at Ifremer(Centre de Brest) with a Cameca SX50. Analyses were generallyobtained at 15 kV accelerating voltage with a sample current of 12 nA.

Major and trace element concentrations were determined using re-spectively, ICP-AES (inductively coupled plasma-atomic emissionspectrometry) and ICP-MS (inductively coupled plasma-mass spec-trometry) in Grenoble according to the procedure described by Barratet al. (2000). The accuracy of major and trace element concentrationsis better than 5% (probably better than 3% for all the REEs), except forW and Cs (10 to 30% for the lowest concentrations) based on variousstandards and sample duplicates.

3. RESULTS AND DISCUSSION

3.1. Petrography and Mineral Compositions

NWA 1240 is an unbrecciated brown rock weighing 98 g andcoated by a dark film of desert varnish. Only a few fusion crustpatches were preserved. Rare thin fractures are filled by terres-trial carbonates. The sawn surfaces display large green pyrox-ene phenocrysts set in a fine-grained groundmass, and a fewvesicles with a maximum diameter of 2 mm.

In polished section, NWA 1240 exhibits a porphyritic textureconsisting of pyroxene phenocrysts (the largest grains are up to4 mm in length) in a fine-grained groundmass. The latterconsists mainly of a variolitic (fan-spherulitic) assemblage offine elongate pyroxene and plagioclase crystals, with a typicallength of 100–150 �m (Fig. 1). Minor phases are skeletalchromite, iron, silica, troilite, ilmenite and minute amounts ofphosphate and fayalite. The modal abundances of minerals aredifficult to establish accurately, but we estimate the area frac-

tion of pyroxene phenocrysts in the studied section to be�30%. Representative analyses of the mineral phases are givenin Table 1.

Pyroxene phenocrysts are solid to skeletal, hollow “sodastraw” in shape, and always fractured (Fig. 1). They containtroilite and Fe-metal inclusions, which are often arranged inplanar arrays, possibly revealing healed fractures. Similarplanes of opaque minerals were described previously in dioge-nitic pyroxenes (e.g., Gooley and Moore, 1976; Benzerara etal., 2002). Small chromite and metal (pure Fe) inclusions arealso present. The phenocrysts are compositionally zoned fromEn76.0Wo1.9Fs22.1 to En43.0Wo9.7Fs47.3 (Fig. 2). The zoning isnormal but the Fe-enrichment toward the rim is somewhatirregular.

Two textural types of pyroxenes are present in the ground-mass: (1) small skeletal crystals (typically greater than 100 �macross) and (2) fine elongate crystals intercalated with plagio-clase. Both types of pyroxene display an extensive normalzoning (Fig. 2) from En62Wo5Fs33 in the cores of the smallskeletal crystals to compositions nearly totally devoid of Mg.The rims are made of Fe-rich pigeonite (Table 1, analysis GP4)and possibly pyroxferroite (Table 1, analysis GP5).

Some Fe-rich rims exhibit symplectic intergrowths (Fig. 3)consisting of Fe-rich augite, ferrosilite (Table 1, analyses GP6and 7), troilite, ilmenite, plagioclase and silica. The rims of the

Fig. 1. Backscattered-electron image of a polished section of NWA1240. (A) Low-Ca pyroxene phenocrysts (gray) are set in a groundmassmade of pyroxene (gray to light gray) and plagioclase (black). (B)Fan-spherulitic texture of elongate pyroxene (gray) and feldspar(black).

3960 J. A. Barrat et al.

plagioclase crystals in contact with these Fe-rich pyroxenescontain systematically small silica and troilite inclusions. Ex-tensive elemental mapping and examination of NWA 1240pyroxene rims at high magnification by field emission gun-scanning electron microscopy revealed no clear evidence offayalite in either the pyroxene or the plagioclase rims. The

origin of these assemblages, never described before in anachondrite, is not yet totally understood. They evoke break-down products of pyroxferroite, or Fe-rich pigeonite, whichwould consist of fayalite, hedenbergite and silica (Lindsley etal., 1972; Lindsley, 1981). As suggested by Buchanan et al.(2000), the breakdown of pyroxferroite or Fe-rich pyroxene ispossibly a two-step process in which the Fe-rich pyroxene (orpyroxenoid) first decomposes into augite and metastable low-Ca, Fe-rich pyroxene. The latter then decomposes into fayaliticolivine and silica. If the cooling rate is too fast, the second stepof this process may not occur and the result would be similar tothe NWA 1240 symplectites (P. Buchanan, personal commu-nication).

The FeO/MnO ratios in the pyroxenes (FeO/MnO � 31�6(1 �), n � 520) are similar to those normally measured ineucrites. The variations in Ti, Al and Cr abundances relative tomg# number (� 100 Mg/(Mg � Fe), atomic) are shown inFigure 4. The compositions of the Mg-rich cores of the phe-nocrysts closely resemble those of diogenitic pyroxenes (e.g.,Mittlefehldt, 1994; Fowler et al., 1994), although the NWA1240 pyroxene cores are significantly richer in Cr. Not surpris-ingly, TiO2 displays incompatible behavior, while Cr2O3 be-haves compatibly. The behavior of aluminum is more complex.The phenocrysts exhibit Al2O3 enrichment from cores (�0.7

Table 1. Electron microprobe analyses (wt%) of representative phases in NWA 1240.a

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O P2O5 Total En Wo Fs

Pyroxene phenocrystsPP1 54.92 0.05 0.68 0.94 14.60 0.47 28.11 0.96 n.d. n.d. 0.05 100.79 76.0 1.9 22.1PP2 54.05 0.05 0.95 1.15 16.17 0.56 26.19 1.18 n.d. n.d. n.d. 100.33 72.5 2.3 25.1PP3 53.52 0.05 1.09 1.24 16.99 0.59 25.22 1.40 n.d. n.d. n.d. 100.13 70.5 2.8 26.6PP4 50.66 0.18 3.18 1.52 20.87 0.77 19.84 2.60 �0.02 n.d. n.d. 99.66 59.4 5.6 35.0PP5 51.60 0.19 1.69 0.77 22.79 0.75 19.66 3.46 �0.03 n.d. n.d. 100.94 56.3 7.1 36.6PP6 49.66 0.10 3.69 1.53 22.36 0.88 17.22 4.02 �0.04 n.d. n.d. 99.49 52.8 8.8 38.4PP7 49.81 0.26 1.35 0.65 28.16 0.98 14.38 4.52 n.d. n.d. n.d. 100.13 43.0 9.7 47.3

Groundmass pyroxenesGP1 48.77 0.25 4.61 1.74 21.50 0.70 16.72 5.43 n.d. n.d. �0.02 99.75 51.2 11.9 36.9GP2 48.75 0.23 0.89 0.43 33.43 0.92 10.14 5.24 n.d. n.d. �0.03 100.05 31.0 11.5 57.4GP3 47.50 0.41 1.43 0.37 35.75 1.16 6.56 6.68 n.d. n.d. �0.04 99.91 20.9 15.3 63.8GP4 46.13 0.63 1.13 0.06 41.52 1.07 2.42 6.78 �0.03 n.d. n.d. 99.76 7.9 15.9 76.1GP5 43.92 1.37 1.76 0.00 44.23 1.16 0.09 6.74 n.d. n.d. �0.03 99.31 0.3 16.3 83.4GP6 44.92 1.78 2.15 0.00 32.55 0.80 0.17 16.83 n.d. n.d. 0.05 99.26 0.6 39.6 59.8GP7 43.28 1.86 0.31 0.02 50.70 1.16 0.72 1.18 n.d. n.d. 0.08 99.31 2.4 2.8 94.8

Plagioclases An Ab OrAv., n � 122 49.14 0.03 30.23 0.03 1.60 0.06 0.30 17.36 0.96 �0.01 �0.02 99.74 81.9 9.1 �0.1

ChromitesC1 0.17 0.60 15.59 50.21 27.52 0.50 5.05 0.05 n.d. n.d. n.d. 99.68 (in px phenocryst)C4 0.47 2.29 13.35 46.92 33.17 0.54 1.81 0.17 n.d. n.d. 0.06 98.78 (in mesostasis)C6 0.07 0.45 14.60 52.62 27.28 0.51 4.90 0.06 �0.02 0.04 0.05 100.60 (in mesostasis)

SilicaS1 99.47 0.08 0.34 n.d. 1.63 n.d. n.d. 0.12 n.d. n.d. n.d. 101.64

S Si Mn Fe Co Ni Cu Zn Total

Groundmass sulfide36.36 0.08 0.04 61.85 0.03 0.08 n.d. n.d. 98.43Sulfide inclusion in a pyroxene phenocryst34.07 0.14 0.08 63.61 0.04 n.d. n.d. n.d. 97.95Groundmass metal (n � 6)�0.02 0.03 0.04 98.61 0.08 0.04 �0.02 n.d. 98.81Metal inclusion in a pyroxene phenocryst

n.d. 0.06 0.03 97.26 0.20 0.13 n.d. n.d. 97.67

a n.d., not detected.

Fig. 2. Quadrilateral pyroxene composition of NWA 1240 py-roxenes. The field of diogenitic pyroxenes is drawn using the data ofMittlefehldt (1994) and Fowler et al. (1994). The field of unequilibratedeucrite Bluewing 001 and clast Yamato 75011,84 are shown for com-parison (Takeda et al., 1994; Warren and Gessler, 2001).

3961NWA 1240 eucrite

wt%) to rims (�1.5 to 4 wt%). Such high concentrations in Alwere previously described in metastable pyroxenes from rap-idly cooled lunar basalts (e.g., Engelhardt et al., 1989). At mg#�60, the groundmass pyroxenes display the highest Al2O3

contents (�4.6 wt%). From mg# � 60 to nearly 0, Al2O3

abundances decrease as a consequence of the onset of plagio-clase crystallization.

Fine elongate tapered crystals of plagioclase intercalatedwith pyroxenes are a major phase in the groundmass where itmakes up �50% of the volume. Excluding the rims containingsilica and troilite inclusions (see above), the crystals are ratherhomogeneous and display only a modest normal zoning, withCa/(Ca � NacK) ratios (atomic) varying between 0.88 and0.93. They always contain significant amounts of Fe (FeO �0.9–2.5 wt%) and Mg (MgO � 0.07–0.5 wt%). It seemsunlikely that these high concentrations were produced partly bya contribution from neighboring Fe-Mg silicates (Fig. 5).Rather, Fe and Mg can substitute for Al in tetrahedral coordi-nation, as previously observed for lunar plagioclase (Longhi etal., 1976, and references therein). The deficiencies of Al � Siin the tetrahedral sites displayed by NWA 1240 plagioclase arecompensated for by Fe and Mg substitutions (Fig. 5). Assuggested for lunar mare basalts (e.g., Longhi et al., 1976), Feand Mg are possibly controlled by the Ca(Mg,Fe2�)Si3O8

plagioclase component. Structural formula calculations indi-cate that NWA 1240 feldspars may contain between 6 and 12mol% of the Ca(Mg,Fe2�)Si3O8 end member, between 78 and86% anorthite, between 7 and 11% albite, and less than 0.5%

orthoclase. Crystallographic studies must be undertaken to bet-ter constrain the crystal structure of NWA 1240 plagioclase.

Chromite is associated with pyroxene crystals as either smallinclusions or dispersed in the groundmass as skeletal crystals�100–400 �m in size. Chromite crystals in contact with thegroundmass are zoned, and their rims contain dispersed inclu-sions of silica (�1 �m). The cr#- ( � 100 Cr/(Cr � Al),atomic) and mg#-numbers vary from, respectively, 58 to 72,and 9 to 27. They are similar in composition to those reportedin diogenites (Mittlefehldt, 1994; Bowman et al., 1999) andpoorer in Ti than eucritic spinels (e.g., Yamaguchi, 2000).

Late-stage intergrowths of iron, troilite, silica, fayalitic oli-vine, augitic pyroxene and plagioclase are present in thegroundmass, which indicate an oxygen fugacity close to theIQF buffer. Phosphate grains occur in the groundmass but arescarce and too small to be properly identified.

Fig. 3. Backscattered-electron image of groundmass pyroxene rimsdisplaying ferrosilite-Fe-rich augite intergrowths. A � augite, Fs �ferrosilite, Pl � plagioclase, Px � pyroxene, Si � silica. The unlabeleddark material included in the symplectites is plagioclase and silica.

Fig. 4. Variations of TiO2, Al2O3, and Cr2O3 relative to atomic 100� Mg/(Fe � Mg) in pyroxenes from NWA 1240. The field of dioge-nitic pyroxenes (D) is shown for comparison (Fowler et al., 1994;Mittlefehldt, 1994). See Figure 2 for the symbols.

3962 J. A. Barrat et al.

Finally, the rare thin carbonate veins cross-cutting the me-teorite are the only evidence of terrestrial contamination. Metaland troilite are not oxidized and silicates display no sign ofweathering.

3.2. Bulk Chemistry

During terrestrial residence, the composition of meteoritescan be substantially modified. Because NWA 1240 is a Saharanfind, it is important to evaluate the possible effects of hot-desertweathering on the compositions of eucrites. We will discussthese effects first before reporting on the composition of NWA1240 composition.

3.2.1. Evaluation of Hot-Desert weathering on eucritecompositions

Over the last two decades, it has been amply demonstratedthat cold desert weathering is able to generate abnormal traceelement distributions in meteorites (Shimizu et al., 1984; Flossand Crozaz, 1991; Mittlefehldt and Lindstrom, 1991, 2003).The most spectacular effects on bulk-rock samples are shownby the rare earth elements (REEs). Phosphates are importantREE-carriers in eucrites (Hsu and Crozaz, 1996) and theseminerals can be partly dissolved in water equilibrated with theatmosphere (Mittlefehldt and Lindstrom, 1991). This may forexample lead to the decoupling of the REE abundances fromthose of the less mobile incompatible elements (such as Ta, Zr,and Hf): the Hf/Sm ratio displayed by unweathered eucrites ischondritic (Hf/Sm � 0.72 � 0.04 [1�, n � 20], Barrat et al.,2000, and this work), but in Antarctic eucrites, weatheringprocesses have produced a wide range of values for this ratio(Hf/Sm � 0.5–3.2, Mittlefehldt and Lindstrom, 1991). More-over, Ce can be oxidized to the �4 state and in this waybecome fractionated from the �3 charged REEs. Conse-quently, weathering of Antarctic eucrites generates a variety ofREE patterns, some exhibiting positive, some negative, Ce

anomalies, and likewise for Eu anomalies. Occasionally, theenrichment in the light REEs is observed as well (Mittlefehldtand Lindstrom, 1991, 2003).

Hot desert weathering is no exception and similar effects,if not more adverse, are to be expected. It is now wellaccepted that the chemical compositions of some Saharanfinds were significantly modified during the relatively fewyears they spent in the desert upon falling (Barrat et al.,1999, 2001; Stelzner et al., 1999; Crozaz and Wadhwa,2001). We analyzed large aliquots (0.3 to 2.8 g) of fresheucrite falls (Table 2, Bereba, Bouvante, Jonzac, Juvinas andSerra de Mage), and a group of noncumulate eucrites col-lected, just like NWA 1240, in Moroccan Sahara (Table 3,Fig. 6), to evaluate the effects of hot desert weathering.Agoult is an unbrecciated granulitic eucrite displaying noevidence of secondary minerals. By contrast, the other Sa-haran eucrites (the monomict eucrites Aoufous, Igdi, North-west Africa 047 (NWA 047) and the polymict eucritesSmara and Northwest Africa 049 (NWA 049)) are visiblyweathered: some surfaces appear rusty and fractures arefilled with calcite. Gypsum occurs in some fractures ofNWA 047. Compared with falls, Saharan finds display strik-ing differences. First of all, Pb concentrations are muchhigher in finds (Pb � 0.48 –1.45 �g/g, Table 3) than in falls(�470 ppb, Manhes et al., 1984; Tera et al., 1997, and thiswork). Similarly, Saharan finds are significantly enriched inBa and Sr compared with falls (Fig. 7). Such enrichment hasalready been observed in numerous Saharan achondrites andis the consequence of precipitation in fractures of secondarycarbonates and sulfates (e.g., Barrat et al., 1999, 2001,2002b; Folco et al., 2000; Zipfel et al., 2000; Fagan et al.,2002). Except for NWA 047, which has an unusually slightlight REE enrichment ((La/Sm)n � 1.1–1.5), no significantweathering effects were observed for the REEs in our dataset (Fig. 8). Moreover, the Hf/Sm ratio is chondritic for allthe noncumulate eucrites analyzed here (Hf/Sm � 0.64 –0.77), indicating limited (if any) REE mobilization in theother samples. Other elements do not exhibit anomalousconcentrations either. These results therefore allow us tosuggest that among the elements we have determined, Pb, Baand Sr are the most sensitive indicators of Saharan weath-ering. U, Cs and the REEs, which are potentially mobile(Barrat et al., 1999, 2001; Folco et al., 2000; Zipfel et al.,2000; Crozaz and Wadhwa, 2001), do not display systematicsubstantial modifications of their concentrations in Saharanmeteorites.

3.3.2. Bulk chemistry of NWA 1240

A 500 mg interior fragment of the NWA 1240 main masswas finely ground using a sapphire mortar and pestle. Thechemical composition was determined using a large fraction ofthe powder (208.8 mg). NWA 1240 is a basaltic rock muchricher in MgO (10.4 wt%) and Cr2O3 (0.71 wt%) than typicalfor noncumulate eucrites, which contain 6–8 wt% MgO and0.2–0.4 wt% Cr2O3. Rather, the major element abundances ofNWA 1240 are similar to Mg-rich clasts from the Kapoeta andYamato 7301 howardites (Table 4). It has been shown thatLunar, Martian, and asteroidal basalts and gabbros display

Fig. 5. Fe � Mg vs. Si � Al in plagioclase from NWA 1240. Theanalyses of the plagioclase of the eucrite Juvinas were obtained usingthe same analytical procedure and are shown for comparison.

3963NWA 1240 eucrite

certain distinctive element ratios that have become widelyemployed as diagnostic tools to identify parent bodies of ex-traterrestrial basalts. As suggested by Treiman et al. (2000), aGa/Al vs. Fe/Mn plot can be used to distinguish eucrites fromlunar basalts, shergottites, nakhlites, angrites and NWA 011, anunusual basaltic achondrite discovered recently (Yamaguchi etal., 2002). As shown in Figure 9, this diagram confirms thatNWA 1240 does indeed belong to the HED family. Thisconclusion is in agreement with other diagnostic ratios such asAl2O3/Na2O (40) and K/La (�330). Furthermore, as shown ina TiO2 vs. FeO*/MgO plot (Fig. 6), the bulk major elementcomposition of NWA 1240 resembles those of cumulate eu-crites.

In contrast with the eucrite finds discussed above, theNWA 1240 fragment analyzed here lacks the chemical fin-gerprints of Saharan alteration: its Pb concentration is verylow (Table 4); Ba and Sr concentrations are not anomalouslyhigh (Fig. 7); the Th/U and Hf/Sm ratios are chondritic

(Th/U � 3.65, Hf/Sm � 0.74). We therefore suggest that theNWA 1240 bulk composition can be used to make inferencesabout its petrogenesis.

Figure 7 compares the trace element concentrations of NWA1240 with other eucrites. Despite its quenched texture, NWA1240 is un-like other noncumulate eucrites which all showhigher incompatible trace element concentrations (e.g., Yb, La,Th, Nb, Zr). By contrast, NWA 1240 displays trace elementabundances similar to those of cumulate eucrites. The resem-blances between NWA 1240 and cumulate eucrites are strikingand extend to the entire REE pattern (Fig. 10), which for bothis relatively flat ((La/Yb)n � 0.76) with a large positive Euanomaly (Eu/Eu* � 1.37).

Finally, concentrations of Co, Cu, Ni, Sc, W and Zn in NWA1240 are similar to those of eucrites in general (e.g., Mit-tlefehldt et al., 1998; Barrat et al., 2000; Quitte et al., 2000) butcannot be used to make a distinction between cumulate andnoncumulate lithologies.

Table 2. Major and trace element compositions of large aliquots of eucrite falls (oxides in wt%, other elements in �g/g). Total Fe is expressed asFeO (FeO*).

Serra deMage-bulk

Berebabulk

Bouvantebulk

Jonzacmatrix

Juvinasmatrix

Juvinasclast 1

Juvinasclast 2

Juvinasclast 3

Juvinasclasts av.

Sample mass 1.06 g 2.11 g 1.47 g 0.389 g 2.34 g 1.81 g 2.82 g 2.69 gTiO2 0.098 0.60 1.15 0.75 0.68 0.65 0.63 0.77 0.69Al2O3 18.42 12.77 12.00 12.28 13.23 13.61 12.88 12.48 12.91FeO* 12.64 21.14 20.36 19.55 18.48 20.21 18.57 19.88 19.46MnO 0.41 0.50 0.48 0.50 0.48 0.48 0.50 0.50 0.50MgO 8.63 6.47 6.50 7.19 6.52 7.40 7.44 6.74 7.17CaO 11.66 10.67 10.33 10.42 10.95 11.27 10.28 10.33 10.54Na2O 0.44 0.55 0.62 0.54 0.53 0.52 0.46 0.48 0.48Li 3.88 9.70 12.93 10.50 9.96 9.24 9.51 9.65 9.50Be 0.06 0.28 0.53 0.30 0.29 0.26 0.27 0.29 0.28Sc 21.6 28.9 27.7 31.8 31.9 32.4 33.6 33.1 33.1Co 8.78 3.09 14.04 11.61 7.26 6.64 7.06 6.20 6.64Ni 4.7 6.0 3.8 4.7 3.5 4.4 11.3 4.0 6.89Cu 2.86 5.09 8.08 7.07 6.90 6.06 6.24 8.07 6.87Zn 1.38 13.81 8.06 6.28 5.55 5.24 5.71 6.29 5.81Ga 1.83 2.11 2.41 2.17 2.34 2.19 2.33 2.40 2.32Rb 0.08 0.22 0.60 0.51 0.18 0.14 0.15 0.16 0.15Sr 73.3 74.0 86.7 73.5 77.5 72.4 78.3 79.0 77.1Y 1.74 16.29 31.18 19.32 18.44 17.51 18.22 22.27 19.53Zr 3.31 41.74 99.32 50.02 47.92 43.80 47.96 59.31 51.10Nb 0.44 3.66 8.01 4.79 4.44 4.00 4.16 5.22 4.51Cs 0.002 0.008 0.022 0.015 0.007 0.005 0.004 0.004 0.004Ba 4.21 27.50 55.62 28.88 30.43 26.07 27.48 33.91 29.49La 0.299 2.45 5.48 2.78 2.73 2.62 2.76 3.41 2.96Ce 0.784 6.51 14.50 7.28 7.27 6.85 7.19 8.92 7.74Pr 0.114 0.994 2.20 1.11 1.10 1.04 1.09 1.34 1.17Nd 0.537 4.83 10.58 5.49 5.35 5.12 5.30 6.74 5.78Sm 0.159 1.57 3.33 1.82 1.77 1.67 1.68 2.12 1.84Eu 0.430 0.584 0.800 0.594 0.628 0.597 0.636 0.654 0.633Gd 0.197 2.09 4.44 2.44 2.40 2.24 2.29 2.89 2.50Tb 0.039 0.385 0.774 0.452 0.436 0.402 0.420 0.521 0.453Dy 0.274 2.61 5.08 3.07 2.92 2.68 2.80 3.55 3.05Ho 0.064 0.575 1.11 0.687 0.643 0.585 0.621 0.777 0.670Er 0.193 1.71 3.13 1.95 1.85 1.67 1.80 2.23 1.92Yb 0.211 1.61 2.81 1.81 1.78 1.61 1.78 2.16 1.87Lu 0.035 0.242 0.426 0.275 0.274 0.242 0.274 0.323 0.284Hf 0.071 1.06 2.45 1.27 1.22 1.15 1.24 1.52 1.32Ta 0.025 0.186 0.431 0.246 0.215 0.215 0.219 0.265 0.235W 0.008 (0.158) 0.235 0.060 0.063 0.063 0.080 0.084 0.077Pb 0.032 0.15 0.39 0.42 0.47 0.20 0.17 0.21 0.19Th 0.027 0.304 0.724 0.333 0.336 0.338 0.338 0.401 0.361U 0.010 0.060 0.174 0.081 0.081 0.079 0.092 0.110 0.095

3964 J. A. Barrat et al.

3.4. Formation of NWA 1240

The texture of NWA 1240 is highly unusual for an achon-drite but strongly resembles the textures of some lunar marelavas, such as the quartz-normative basalts from the Apollo 15landing site. As demonstrated by experimental studies (Lofgrenet al., 1975; Grove and Walker, 1977), this particular type oftexture indicates a high cooling rate. Moreover, basalts cooledto below 900°C within 3 d may preserve pyroxferroite (Lind-sley et al., 1972). If the cooling rate is lower, pyroxferroite isdecomposed to a stable assemblage of fayalite, hedenbergite,and silica, which so far has not been observed in NWA 1240.Therefore, the preservation in the rims of some NWA 1240groundmass pyroxenes of metastable pyroxenes and probablypyroxferroite, testifies to a fast cooling rate. Furthermore, theextremely unequilibrated character of NWA 1240 indicates thatun-like most of the HED meteorites, this rock was not meta-morphosed.

Because NWA 1240 displays a porphyritic texture, one mayargue that the high MgO and Cr2O3 contents displayed by thebulk analysis (Table 4) are produced by pyroxene (and chro-mite) accumulation, or that the analyzed fragment was toopyroxene-rich and not representative of the whole meteorite.The apparent KD(Fe/Mg) ( � ([FeO]px/[FeO]br)/([MgO]px/[MgO]br)) between the phenocryst cores (px) and the bulk rockcomposition (br) can be calculated to assess these possibilities.Using the most Mg-rich pyroxene core analysis (Table 1,analysis PP1), a KD(Fe/Mg) value of 0.27 is obtained. Thisresult is similar to the experimental KD(Fe/Mg) measured forlow-Ca pyroxenes in eucritic systems (e.g., KD(Fe/Mg) � 0.30[Stolper, 1977]; KD(Fe/Mg) � 0.27 [Bartels and Grove, 1991]).The behavior of Cr during pyroxene fractionation is in part afunction of oxygen fugacity and melt composition (e.g., Groveand Bence, 1977; Stolper, 1977). Apparent DCr ( � 1.3) forNWA 1240 pyroxene is in the range of the experimental values

Table 3. Major and trace element compositions of Saharan eucrites Agoult, Aoufous, Igdi, NWA 047, NWA 049 and Smara (oxides in wt%, otherelements in �g/g).

# Mass-mg

AgoultbulkAGO116.8

AoufousbulkAOU119.5

IgdibulkIGD163.3

N 047bulkB47

165.0

N 047matrixM47164.2

N 047clastC47

213.0

N 049clastC49

170.5

SmaragabbroSG1174.5

SmaragabbroSG2167.6

SmaramatrixSM1138.8

SmaramatrixSM2192.6

SmaramatrixSM3224.4

SmaramatrixSM4123.3

TiO2 0.62 0.63 0.93 0.65 0.71 0.74 0.67 0.67 — 0.79 0.85 0.85 0.82Al2O3 13.14 12.74 11.81 12.23 12.56 12.65 11.74 12.28 — 12.19 12.30 11.18 10.47FeO* 17.84 18.07 20.55 20.57 20.26 19.79 18.23 17.46 — 17.14 18.46 18.86 17.57MnO 0.56 0.57 0.57 0.62 0.62 0.59 0.58 0.58 — 0.62 0.61 0.58 0.53MgO 7.77 6.93 5.53 7.07 7.14 6.77 7.05 8.13 — 6.92 6.82 7.29 7.21CaO 11.40 10.52 11.76 10.44 10.97 10.60 10.09 10.89 — 11.40 10.51 12.31 12.39Sc — 32.4 32.9 31.4 27.4 31.4 33.8 24.2 26.5 28.9 25.6 26.8 25.9Co 8.74 7.65 9.25 5.24 5.84 6.60 5.74 7.66 7.87 5.98 6.42 10.19 7.53Ni 5.0 — 48.0 7.1 — — — 5.0 — — — 17.7 —Ga 2.01 — 2.16 1.80 — — — 1.77 — — — 1.69Rb 0.18 0.42 0.60 0.35 0.29 0.54 0.17 0.44 0.60 0.74 0.55 0.78 0.66Sr 87.2 109 110 77.7 77.6 84.4 94.2 82.6 117 117 101 101 107Y 17.24 18.03 25.90 18.13 16.70 20.06 17.75 17.17 21.44 22.94 20.49 21.06 23.16Zr 40.1 42.7 68.3 46.3 50.3 55.4 44.5 50.9 63.6 67.4 62.8 61.4 66.3Nb 3.83 3.60 5.30 3.43 3.84 4.17 3.59 3.58 4.81 5.13 5.60 4.47 4.81Cs 0.010 0.019 0.022 0.015 0.011 0.019 0.010 0.011 0.021 0.026 0.017 0.030 0.021Ba 30.3 72.5 107 38.9 36.2 39.0 58.7 45.6 54.8 125 124 67.9 92.1La 2.21 2.80 3.94 4.74 3.90 3.54 2.68 2.79 3.85 3.93 3.49 3.65 4.09Ce 5.81 7.33 10.34 11.09 9.00 8.88 6.90 7.38 10.08 10.30 9.13 9.67 10.61Pr 0.89 1.10 1.57 1.49 1.21 1.29 1.05 1.11 1.50 1.55 1.37 1.45 1.58Nd 4.39 5.36 7.66 6.72 5.66 6.19 5.13 5.54 7.37 7.54 6.78 7.09 7.86Sm 1.48 1.71 2.47 1.95 1.74 1.99 1.62 1.75 2.24 2.39 2.10 2.19 2.48Eu 0.602 0.627 0.717 0.625 0.613 0.629 0.544 0.616 0.724 0.660 0.622 0.617 0.683Gd 2.03 2.35 3.33 2.51 2.25 2.64 2.26 2.31 2.85 3.13 2.83 2.87 3.22Tb 0.384 0.421 0.605 0.458 0.412 0.487 0.407 0.425 0.514 0.559 0.508 0.515 0.577Dy 2.62 2.87 4.07 3.10 2.77 3.30 2.78 2.82 3.36 3.69 3.37 3.43 3.80Ho 0.575 0.628 0.888 0.671 0.605 0.732 0.605 0.616 0.733 0.810 0.727 0.745 0.833Er 1.69 1.81 2.57 1.94 1.77 2.12 1.75 1.76 2.08 2.33 2.07 2.13 2.33Yb 1.63 1.75 2.41 1.92 1.73 2.01 1.70 1.64 1.89 2.11 1.92 2.00 2.16Lu 0.255 0.273 0.362 0.293 0.267 0.308 0.260 0.252 0.286 0.320 0.296 0.298 0.330Hf 1.03 1.12 1.73 1.25 1.34 1.45 1.11 1.32 1.55 1.76 1.57 1.62 1.74Ta 0.19 0.19 0.27 0.19 0.23 0.25 0.20 0.20 0.27 0.30 0.31 0.26 0.28W 0.02 0.06 0.11 0.03 0.06 0.06 0.08 0.06 0.07 0.14 0.16 0.13 0.07Pb 1.46 1.10 1.14 1.01 1.00 0.95 0.84 0.48 1.09 1.21 1.04 1.01 1.00Th 0.261 0.360 0.488 0.376 0.342 0.401 0.334 0.358 0.484 0.509 0.441 0.491 0.534U 0.074 0.095 0.137 0.082 0.073 0.130 0.081 0.117 0.174 0.188 0.140 0.174 0.185FeO/MnO 31.86 31.70 36.05 33.18 32.68 33.54 31.43 30.10 — 27.64 30.26 32.52 33.15Hf/Sm 0.70 0.65 0.70 0.64 0.77 0.73 0.69 0.75 0.69 0.74 0.75 0.74 0.70(La/Sm)n 0.94 1.03 1.01 1.53 1.41 1.12 1.04 1.00 1.08 1.03 1.05 1.05 1.04Th/U 3.52 3.80 3.56 4.60 4.69 3.08 4.13 3.06 2.78 2.70 3.15 2.82 2.88

3965NWA 1240 eucrite

obtained under reducing conditions (Grove and Bence, 1977;Stolper, 1977; Bartels and Grove, 1991; Jurewicz et al., 1993,1995; Boesenberg and Delaney, 1997) and indicates that thehigh Cr content of the bulk rock is reasonable. Consequently,an overrepresentation of pyroxene phenocrysts or chromite inthe analyzed fragment is unlikely. The composition of the meltfrom which NWA 1240 crystallized, must have been very closeto the bulk rock analysis given in Table 4.

Positive Eu anomalies in eucrites, and more generally inbasaltic rocks, are usually explained by plagioclase accumula-tion. However, this process cannot account for the large Euanomaly displayed by NWA 1240 as this meteorite is totallydevoid of plagioclase phenocrysts. Alternatively, the unusualREE pattern could be due to under-sampling of Ca-phosphate,but this is unlikely because of the relatively large size (0.5 g) ofthe sample used for the analysis. Consequently, this feature,although completely unexpected for a magmatic liquid, must bea genuine characteristic of the parental melt of NWA 1240.Basaltic liquids displaying significant positive Eu anomaliesare uncommon. On Earth, true basaltic melts with the sameEu/Eu* as NWA 1240 (Eu/Eu* �1.4) are unknown. Only a fewbasaltic glasses with Eu/Eu* slightly higher than 1.10 havebeen analyzed (e.g., Barrat et al., 1993). In the case of lunarrocks, basaltic liquids showing a significant positive Eu anom-aly are unknown as well. The only known melts with a positiveEu anomaly were produced by impact on an anorthosite-richtarget (e.g., Papike et al., 1998, and reference therein). HEDmeteorites are no exception to terrestrial and lunar basalts andthe only marked positive Eu anomalies among HEDs are in

cumulate eucrites (Fig. 10). The strong chemical similaritiesbetween NWA 1240 and cumulate eucrites thus strongly sug-gest that NWA 1240 is an impact melt of a cumulate eucrite-dominated target. The conspicuous lack of relict phases and theparticular texture displayed by NWA 1240 further imply thatthe target was completely melted by the impact (Lofgren,1977).

Two possible types of targets can be considered: (1) assuggested by the low Al2O3 and CaO contents, NWA 1240 mayhave been derived from a melted mixture of diogenite andcumulate eucrite clasts (in other words, an unusual howardite),or alternatively (2) it may be an impact-melted cumulate eu-crite. To discuss these hypotheses, mixtures made of typicaldiogenite and cumulate eucrites (e.g., Mittlefehldt et al., 1998,and references therein) were calculated. Unfortunately, no sat-isfactory match was obtained, probably because our knowledgeof cumulate eucrite chemistry is based on too limited a numberof meteorites. Furthermore, such rocks are extremely hetero-geneous and their bulk compositions are generally difficult toestablish accurately. Thus, the two possible types of targetscannot be distinguished using the available data.

The siderophile element contents of NWA 1240 are low(e.g., Ni � 5.4 �g/g) and indistinguishable from those mea-sured in the unbrecciated cumulate eucrites Serra de Mage,Moama and Moore County (this work and Palme et al., 1978;Mittlefehldt, 1979; Barrat et al., 2000; Mittlefehldt and Lind-strom, 2003). This amount of Ni corresponds to �0.05% CM or

Fig. 6. TiO2 vs. FeO*/MgO plot for NWA 1240 and Saharan eu-crites. The howardite, cumulate and noncumulate eucrite fields weredrawn using data from Barrat et al. (2000), Buchanan et al. (2000),Christophe Michel-Levy et al. (1987), Duke and Silver (1967),Fukuoka et al. (1977), Ghosh et al. (2000), Laul and Gosselin (1990),Ma and Schmitt (1979), McCarthy et al. (1973), Metzler et al. (1995),Mittlefehldt (1979), Palme et al. (1978, 1988), Shukla et al. (1997),Stolper (1977), Wanke et al. (1972, 1974, 1977), Warren and Jerde(1987), and Warren et al. (1990). Fig. 7. Trace elements vs. Yb for eucrites. The data set is limited to

analyses obtained by ICP-MS using the same calibration strategy(Barrat et al., 2000, and this study) to avoid analytical bias.

3966 J. A. Barrat et al.

CR chondrite contamination (based on the averages given byLodders and Fegley, 1998), and indicates that (1) NWA 1240was not contaminated by the projectile during the impact melt-ing event and (2) the target material did not experience signif-icant chondritic projectile contamination before the impactmelting event.

5. CONCLUSIONS

NWA 1240 is an anomalous unbrecciated eucrite, whichshows remarkable mineralogical and geochemical features.First, it displays an unusual porphyritic texture indicating ahigh cooling rate. Second, it was not affected by metamorphicevents. Its pyroxene crystals are unequilibrated and show oneof the widest ranges of compositions ever described for aeucrite, from En76.0Wo1.9Fs22.1 to compositions nearly devoidof Mg (pyroxferroite and unusual ferrosilite and Fe-augitesymplectites). Third, plagioclase crystals contain significantamounts of Fe and Mg which are possibly controlled by theCa(Mg,Fe2�)Si3O8 plagioclase component. Further studiesmust be undertaken to better constrain the origin of the Fe-

augite and ferrosilite symplectites and the crystal structure ofNWA 1240 plagioclase.

The bulk chemical composition of NWA 1240 exhibits strik-ing similarities with cumulate eucrites, such as similar incom-patible trace element concentrations and a significant positiveEu anomaly (Eu/Eu* � 1.37). This suggests that NWA 1240,in spite of its quenched texture, did not form from a “primitive”eucrite melt but rather represents an impact melt from a cumu-late eucrite-dominated target.

High siderophile element abundances have often been usedas the criterion for identifying possible impact melts, especially

Fig. 8. REE patterns of Saharan eucrites. The reference chondrite isfrom Evensen et al. (1978).

Table 4. Major and trace element compositions of achondrite NWA1240 and literature magnesian eucrites (oxides in wt%, other elementsin �g/g).a

NWA1240 This

work

EET92014,7

ML

KapoetaclastML

Kapoetaclast D

Y7301clast PE1

IT

SiO2 49.2 49.8 49.47 48.37TiO2 0.31 0.81 0.47 0.78 0.44Al2O3 9.99 11.5 10.4 9.88 10.54Cr2O3 0.71 0.48 0.54 0.9 0.49FeO* 20.19 18.5 16.5 18.77 19.33MnO 0.57 0.53 0.5 0.54MgO 10.36 8.40 12.1 10.1 11.44CaO 8.12 9.18 7.9 9.34 8.52Na2O 0.25 0.43 0.28 0.47 0.35K2O �0.03 0.06 0.04 0.03P2O5 0.09 0.11 0.09Li 4.12Be 0.08Sc 29.5 27.1V 130Co 7.5 3.14Ni 5.4Cu 4.4Zn 2.5Ga 2.03Rb 0.06Sr 42Y 5.83Zr 13.21Nb 1.09Cs 0.010Ba 16.32 63La 0.764 5.63Ce 1.98 16Pr 0.301Nd 1.51Sm 0.489 3.29Eu 0.259 0.8Gd 0.687Tb 0.128 0.75Dy 0.917Ho 0.213Er 0.635Yb 0.680 2.44Lu 0.111 0.358Hf 0.36 2.35Ta 0.06 0.34W 0.14Pb 0.09Th 0.095 0.65U 0.026 0.16

a D � Dymek et al. (1976); ML � Mittlefehldt and Lindstrom(1997); IT � Ikeda and Takeda (1984).

3967NWA 1240 eucrite

in the case of lunar samples (e.g., Papike et al., 1998). The Coand Ni contents of NWA 1240 are low and thus demonstrate, infull agreement with Mittlefehldt and Lindstrom (1997), that

lack of chondritic siderophile element contamination is notnecessarily a firm indication of whether any given sample ispristine.

Acknowledgments—Samples were generously provided by BrunoFectay and Carine Bidaut (Agoult, Aoufous, Igdi, Smara, NWA 049and NWA 1240), Alain Carion (NWA 047) and the Museum Nationald’Histoire Naturelle de Paris (Bereba, Bouvante, Jonzac, Juvinas, Serrade Mage). The help of Michelle Denise (MNHN) was greatly appre-ciated during the preparation of the MNHN samples. We thank DavidMittlefehldt for the editorial handling, Paul Buchanan, Marc Norman,Akira Yamaguchi for constructive comments, and Pascale Barrat forher help. We gratefully acknowledge the Programme National dePlanetologie (INSU) for financial support. This research has made useof NASA’s Astrophysics Data System Abstract Service.

Associate editor: D. W. Mittlefehldt

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